Among permanent magnets, R-T-B type magnets exhibit a high maximum magnetic energy product and are being used for HD (hard disk), MRI (magnetic resonance imaging), various types of motors and the like by virtue of their high-performance characteristics. A recent increase in demand for energy saving, in addition to enhancements in the heat resistance of R-T-B type magnets, has caused the usage rate in motors, including automobile motors, to increase.
R-T-B type magnets may comprise Nd, Fe and B as the main components and therefore, the magnets of this type are collectively called an Nd—Fe—B type or R-T-B type magnet. In an R-T-B type magnet, R is primarily Nd with a part being replaced by another rare earth element such as Pr, Dy and Tb, or, more generally, R is at least one member selected from rare earth elements including Y; T is Fe with a part being replaced by a transition metal such as Co and Ni; and B is boron and may be partially replaced by C or N. Also, in R-T-B type magnets, one species or a combination of a plurality of species selected from Cu, Al, Ti, V, Cr, Ga, Mn, Nb, Ta, Mo, W, Ca, Sn, Zr, Hf and the like may be added as an additive element.
The R-T-B type alloy which can be used in an R-T-B type magnet is an alloy where a ferromagnetic R2T14B phase contributing to the magnetization activity is the main phase and coexists with a nonmagnetic, rare earth element-enriched and low-melting point R-rich phase. This alloy is an active metal and therefore, generally melted or cast in vacuum or in an inert gas. From the cast R-T-B type alloy ingot, a sintered magnet is usually produced by a powder metallurgy process as follows. The alloy ingot is ground into an alloy powder of about 3 μm (as measured by FSSS (Fisher sub-sieve sizer)), press-shaped in a magnetic field, sintered at a high temperature of about 1,000 to 1,100° C. in a sintering furnace, then subjected to, if desired, heat treatment and machining, and further plated for enhancing the corrosion resistance, thereby completing a sintered magnet.
In the R-T-B type sintered magnet, the R-rich phase plays the following important roles:
1) becoming a liquid phase at the sintering by virtue of a low melting point and thereby contributing to high densification of the magnet and in turn, enhancement of the magnetization;
2) eliminating unevenness on the grain boundary and thereby yielding reduction in the nucleation site of the reversed magnetic domain and increase in the coercive force; and
3) magnetically isolating the main phase and thereby increasing the coercive force.
Accordingly, if the R-rich phase in the shaped magnet is in a poorly dispersed state, it incurs local failure of sintering or reduction of magnetism. Therefore, it is important that the R-rich phase is uniformly dispersed in the shaped magnet. Here, the R-rich phase distribution is greatly affected by the texture of the raw material R-T-B type alloy.
Another problem encountered in casting an R-T-B type alloy is production of α-Fe in the cast alloy. The α-Fe has deformability and remains in the grinder without being ground, and this not only decreases the grinding efficiency at the grinding of alloy but also affects the compositional fluctuation or particle size distribution. If α-Fe still remains in the magnet after sintering, reduction in the magnetic characteristics of the magnet results. Accordingly, α-Fe has been dealt with as a material which should be eliminated from the raw material alloy as much as possible. For this purpose, an alloy has been heretofore subjected to a homogenization treatment at a high temperature for a long time to eliminate α-Fe. When the amount of α-Fe in the raw material alloy is small, this may be removed by a homogenization heat treatment. However, α-Fe is present as a peritectic nucleus and therefore, its elimination requires solid phase diffusion for a long time. In the case of an ingot having a thickness of several cm and a rare earth content of 33% or less, elimination of α-Fe is practically impossible.
In order to solve the problem that α-Fe is produced in the R-T-B type alloy, a strip casting method (simply referred to as an “SC method”) of casting an alloy ingot at a higher cooling rate has been developed, and this method is being used in actual processes.
The SC method is a method of solidifying an alloy through rapid cooling, where a molten alloy is cast on a copper roll of which the inside is water-cooled, and a flake of 0.1 to 1 mm is produced. In the SC method, the molten alloy is supercooled to the temperature where the main R2T14B phase is produced, so that an R2T14B phase can be produced directly from a molten alloy and the precipitation of α-Fe can be suppressed. Furthermore, in the SC method, the alloy comes to have a fine crystal texture, so that an alloy having a texture allowing for fine dispersion of an R-rich phase can be produced. The R-rich phase expands by reacting with hydrogen in a hydrogen atmosphere and becomes a brittle hydride. By utilizing this property, fine cracking commensurate with the dispersion degree of the R-rich phase can be introduced. When an alloy is pulverized through this hydrogenation step, a large amount of fine cracks produced by the hydrogenation trigger breakage of the alloy and therefore, very good grindability is attained. The internal R-rich phase in the alloy produced by the SC method is thus finely dispersed, and this leads to good dispersibility of the R-rich phase also in the magnet after grinding and sintering, thereby succeeding in enhancing the magnetic characteristics of the magnet (see, for example, Patent Document 1).
The alloy flake produced by the SC method is excellent also in terms of texture homogeneity. The texture homogeneity can be compared by the crystal grain diameter or the dispersed state of R-rich phase. In the case of an alloy flake produced by the SC method, a chill crystal is sometimes generated on the casting roll side of the alloy flake (hereinafter referred to as a “mold face side”), but an appropriately fine homogeneous texture yielded by the solidification through rapid cooling can be obtained as a whole.
As described above, in the R-T-B type alloy produced by the SC method, the R-rich phase is finely dispersed and the precipitation of α-Fe is also suppressed, so that in the production of a sintered magnet, the homogeneity of the R-rich phase in the final magnet can be increased and the adverse effect of α-Fe on the grinding and magnetism can be prevented. In this way, the R-T-B type alloy ingot produced by the SC method has an excellent texture for the production of a sintered magnet. However, along with enhancement of characteristics of the magnet, demands for high-level control of the raw material alloy texture, particularly, the presence state of the R-rich phase, are increasing.
The present inventors have previously made studies on the relationship between the texture of the cast-produced R-T-B type alloy and the behavior at the hydrogen cracking or pulverization and found that, in order to control the particle size of the alloy powder for a sintered magnet, the control of the dispersed state of R-rich phase is important (see, for example, Patent Document 2). Also, it has been found that fine division readily occurs in the region where the R-rich phase produced on the mold face side in the alloy (fine R-rich phase region) is extremely finely dispersed, as a result, the grinding stability of the alloy is deteriorated and at the same time, the particle size distribution of the powder is broadened. This finding leads to an understanding that reduction of the fine R-rich phase region is necessary for the enhancement of characteristics of the magnet.
However, even in the R-T-B type alloy disclosed in Patent Document 2, more enhancement of the magnetic characteristics is required.
Patent Document 1: JP-A-5-222488 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)
Patent Document 2: JP-A-2003-188006